UDM router has "no internet connection" when plugged into BGW320-500 port (Giga at bottom)

Newly installed BGW320-500 connects to the Internet, but I can't get my Ubiquiti Dream Machine (UDM) to reach the Internet when I plug it into the gigabit port (bottom port on BGW, there is a green light showing continuity).  Here's what I've tried:

1) just plugging in UDM and sending it...didn't work, but it was worth a try because that is all I had to do with our old CenturyLink fiber gateway/modem.

2) Set BGW to passthrough mode "DHCPS-fixed" selected the UDM from Passthrough Fixed MAC Address (it is the only wired connection), left DHCP lease at 10 minutes.  On Firewall Status: Packet Filter ON, IP Passthrough ON, NAT default server OFF, Firewall advanced OFF.  Some observations:

-- the BGW has a valid Internet IP address (not static, didn't pay for that)

-- the BGW says the UDM has IP 192.168.1.2 and the gateway is on 192.168.1.254 (web interface)

-- the UDM says it's IP address is 192.168.1.1 (it is configured as 192.168.1.0/24  that's how I had it with Century Link)

-- if I plug a laptop into the BGW I can ping 192.168.1.254, but I can't ping 192.168.1.1 or .2 any other address up to 10 (didn't try above that). 

-- if I plug a laptop into the UDM I can't ping 192.168.1.254. 

My goal is to run my network on the UDM (with four switches) and only have the BGW wifi or ports for troubleshooting when my wife says the UDM network is down.  I assume Passthrough is the best for this, but I'll admit I don't understand what it is doing even after reading up on it -- it seems like the BGW has the Internet IP (WAN) and the UDM gets an internal 192.168.1.x IP where the BGW sends traffic it didn't originate to that IP for the UDM to handle.  Not sure if that's right. 

Anyway, any advice for getting to to work is much appreciated.